Loud speaking telephine instrument



G. M. GIANNINI LOUD-SPEAKING TELEPHONE INSTRUMENT Feb. 20, 1951 3 Sheets-Sheet 1 Original Filed July 2, 1945 Inventor By GABRIEL M. GIANNINI m Attorneys Feb. 20, 19 51 G. M. GlANNlNI 2,542,922

LOUD-SPEAKING TELEPHONE INSTRUMENT Original Filed July 2, 1945 3 Sheets-Sheet 2 FIG. 2

Q g N 'Invemor N) r0 H) y GABRIEL M. GIANNINI Affornevs Feb. 20, 1951 G. M. GIANNINI 2,542,922

LOUD-SPEAKING TELEPHONE INSTRUMENT Original Filed July 2, 1945 1 T 5 Sheets-Sheet 3 FIG. 4

FIG. 7 407 Invenror B GABRIEL M. GIANNINI Afforneys The present invention relates generally to sub- Patented Feb. 20, 1951 PATE NT OFFICE I LOUD SPEAKINGTELEPHON E INSTRUMEN T Gabriel M. Giannini, Pasadena, Calif., assignor, by mesne assignments, to Automatic Electric I 7, Laboratories, Inc., "Chicago, III., a corporation of Delaware Original amnion 1.11,, .2, 1945, Serial No.

602,710. Divided and this application December 13, 1946, Serial No. 716,023 r 6 Claims.-

station apparatus and more particularly to improvements in methods of and apparatus for in- I creasing the stability and efficiency of operation or substation apparatus utilized in loudspeaking transmission systems. The present application is a division of the copending application of Gabriel M. Giannini, Serial No. 602,710 filed July It is an object of the present invention to provide an improved substation instrument for use in two-way transmission systems, wherein a microphone, a loudspeaker and other instrumentalities, such as a-dial and a ringer, are con- -veniehtlymounted and wherein insulating arrangements are provided to isolate acoustically the microphone and the loudspeaker.

- According to another object of the invention the microphone, the loudspeaker and the other instrumentalities of the substation instrument are mechanically isolated from each other in order to minimize mechanical or conductive coupling therebetween through the substation instrument.

It is still another obj :(st of the invention to provicle an improved substation instrumentwhich includes a main casing, adapted to house the various substation instrumentalities, and a plumitting and'the receiving elements. I

v The invention, both as toits organization and m1thod of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in "connection with the accompanying drawings in which Figure 1 is aperspective view of one form of. a loudspeaker-microphone substation instrum nt .which may beused at each of the stations of a transmission system; Fig. 2 is a. sectional view of the substation instrument of Fig. 1 showing the usual instrumentalities including the "loudspeaker, the microphone; the ringer and the dial; .and Figs. 3 to '7, inclusive, illustrate the details of the microphone and loudspeaker assemblies provided at each of the substations of the system. Referring now to Figs. 1, 2 and 3 of the drawings, the substation instrumentthere illustrated. comprises a casing 300 provided with four substantially perpendicular walls which converge and terminate in a convexed, forwardly sloping +30 ing 300.

(Cl. 179-102) 2 L. the diaphragms of the loudspeaker I05'and microphone I04 housed within the casing 300. Fulfthermore, the loudspeaker and microphone units are provided with individual subcasings, the for.- "5 mer subcasing constituting an integral part 'of the main casing 300 and the latt r subcasingconstituting a separate demountable casing whereby the microphone unit and its associated casing may be readily removed from the main casing ill 300. In order to further minimize acoustical coupling between the; rear surfaces of the diaphragms of the twotranslating units, the loud- :speaker and microphone subcasings are filled with acoustical damping material, such, for ext- 115 ,ample, as feltr r j Y More sp cifically considered, the main casing :300 comprises a slightly inwardly sloping front wall portion 30L a substantially perpendicular rear wall portion 302, and a pair of substantially '20 perpendicular side wallportions, the right side wall-303 being illustrated in Fig. 1. The top wall portion 304 is somewhat convexed andis slightly sloped from the higher rear wall 302 toward the lower front wall 30I and converges 25 with all the wallportions of the casing 300. -A plurality of apertures 305 and 306 are also pro; vided in the casing 300 and constitute sound openings for the microphone I04 and loudspeaker I05, respectively, mounted within the main cas- A dialing mechanism 301' is secured in an opening in the top surface of the casing wh-:n the substation instrument is connected to an automatic telephone system, or in the event the substation instrument is connected to a man .-135 ual telephone system a blank escutcheon plate may be substituted in place thereof. For the purpose of permitting sound waves developed during the operation of the ring'r 352, mounted within the casing 300, to be transmitted to the Q0 surrounding, air, a plurality of rectangularly shaped slots 309 are provided in spaced-apart relation along the lower perpendicular side walls of the casing 300. A plurality of decorative strips 3 I 0 are secured to th, surface of the casing, which 45 strips may be formed, for example, of highly polished or plated metal or of colored plastic mari l- I m z- The casing300 may be formed of an insulating material or it may be formed 'of a metal 150 and'is preferably moulded in one piece. In formingthe casing 300 the bottom thereof is left open in order to facilitate the mounting of the Vail-- ous elements of the substation apparatus within the chamber defined by the above-described wall structure. .a base plate .31 I. ,b ine ,p v de cr 3 closing the chamber defined by the walls of the casing. The casing see is also formed with a cylindrical subcasing 312 which extlnds downwardly from the top wall of the main casing 38!] to a point substantially-midway between the top and the bottom thereof and :formed integrally therewith so that a portion of the cylindrical wall forms a portion of the rear wall 382 and ta portion of the side Walls 3113 oithe casing .3110. The bottom of the subcasin'g 312 is also left open in order to facilitate the mountingof the loudspeaker unit H15 within the -1cavity formed by the subcasing 3l2 with the loudspeaker diathe top of the main casing 300,.- .A cover plate.

the cylindrical wall of the subcasing 3l2.

In order to prevent the diaphragm of the loudspeaker unit I05 from being damaged, 'a protective screen 3M of silk or fine mesh metal wire is provided in the opening 3H6 and secured :place by cementing the same to the peripheral edge portion of the opening or by merely clamping the ame in place under pressure. The rubber gasket 8'15 placed between the diaphragm supporting ring and the screen 3% placed over the opening 305' in the casing 300 efiectively prevents passage of air or sound waves around the edge of the loudspeaker diaphragm irom the outside to the inside, or v-i-ce'versa, of the chamber formed by the subcas'ing M2. The rubber gasket 2315 also prevents sound waves originating at one surface of the loudspeaker diaphragm from impinging upon the other surface thereof. In order *to prevent acoustical coupling between the rear surface @of the loudspeaker and the microphone diaphragms, and to prevent undesirable resonance effects within the chamber form-ed by' the subicasing 312,-a suitable amount of acoustical insul-ating material such as, for example, a plurality of compressible felt rings 3 16 are provided, which substantially surround the loudspeaker unit M5.

The loudspeaker unit 105 is mounted upon the cover plate 313 by a plurality of screws 3M, one of which is shown in Fig. 2, and tor "the purpose of preventing the cover plate 313 .and any other sous structure mechanically connected thereto from transmitting vibrations either to or from the loudspeaker unit Hi5, a mounting assembly of the cushion type is provided. More specifically, the annular recess provided in the soft rubber 'jgromrnet 31 8a snug-1 engages the adjacent surfaces or the opening formed in the cover plate "313 and a mounting screw 31"! having a metal Washer 3 9 and a locking washer 32 extends iih-rougha central opening provided in the gromgme't '3 l8a and through the central opening of the soft rubber grommet spacer 348i), and threaded into a tapped hole provided infthe loudspeaker imit 4 05. Preferably three such cushion type mountings are provided to resiliently secure the "loudspeaker unit upon the cover plate 3 I3.

For the purpose of securing the cover plate 313 to the subcasing 31 2, a plurality of equally spaced tapped holes are provided into which the screws 322a, having locking washers 322b, are threaded after they have been inserted in corresponding holes provided in the cover plate 3l3. A soft-rubber gasket 323 is placed between the jcover platesw and the bottom surface of the 'siibcasing 312', whereby the screws 322a may be utiil'zed to draw the loudspeaker unit U35 upfward'ly to securely clamp the flexible soft rubber gasket 315 between the diaphragm supporting ga 0f th loudspeaker unit H and the under 7 cally, a protective screen 330 of silk or finemesh' 4 surface adjacent the sound opening 306 in the main casing 300. The screws 322a are also ef- 'fective to securely clamp the soft rubber gasket 323 between the cover plate 3E3 and the bottom surface of the subcasing M2. The soft rubber gasket :3 23 :functions as :a seal -between the cover plate BB and the subcasing 312 and prevents, in

' combination with the acoustical insulating ma- V 313 is provided for closing the-cavity idefinedby terial 3H5, sound waves emanating from the rear 'sur'face'of't'he loudspeaker diaphragm from passing to the front surface thereof and from passing 7 to the microphone diaphragm. The density of ithe acoustical material 3 l6 surrounding the loudspeaker "unit M15 may be altered in order to vary the frequency response of the loudspeaker dia phragm. Thus the frequency response characteristics or the loudspeaker are enhanced and troublesome acoustical coupling between the sound waves created'by the rear surface of the loudspeaker diaphragm; and the microphone is prevented- ,Sirrceithein and out wires (not-shown) forming the yoicecoil of the loudspeaker unit 10.5 are somewhat fragile, they are .soldered to terminals secured to the diaphragm support. .A pair of heavier stranded lead-in wires are also soldered it) the terminals, as will be described hereinafter,

, in order to provide somewhat flexible electrical connections between the voice coil wires and the exterior of the .subcasing 312. In order to fa- :oiiitate electrical connection between an incoming amplifier channel .and the stranded leadain wire extending to the voice coil terminals, a ter- :minal block 3-24 of insulating material has been provided whereby the above -mentioned stranded lead-in Wires ma be secured thereto ior electrical connection with the conductors extending to "the incoming channel amplifier (not shown). More specifically, the terminal block 324mmyprises a plurality of terminals 324a, securedthereto in any suitable manner, which are adapted to accommodate the terminal screws 32%. The terminal block 324 is held in spaced relation upon the cover plate 3l3 by the metal spacers .325 and the -.screw-s 322b, which are somewhat longer than the cover plate securing screws 322a, :and are threaded into tapped holes in the bottom surface of the subcasih-g 3| 2 to secure the terminal block and the cover plate .in position. The above-mentioned insulated wires (not shown) are inserted throu h a central openingprovided in :the soft rubber grommet 326, which snugly engages the adjacent surfaces of the opening formed in the cover plate M3, .and are directl secured to the terminals 324a by means of the screws 324b, In order torcompletely seal the cavity ziormed by the subcasing 312 and to prevent sound waves from passing through the central opening in the grommet .32 6, said opening may The sealed with sealing wax or the like.

. from the foregoing description it will be understood that the loudspeaker unit I05 is mount- ;ed in a sub-casing 3-12 which is a part of and integral with themaincasing 390. Furthermore, it be understood that although sound Waves created at the rear surface of the loudspeaker diaphragm are completely isolated from sound waves created by the front surface thereof and vice versa, the loudspeaker unit may be removed from and replaced in the subcasing 312 in order a than wire is provided in the opening 305} and secured in place by cementing the same to the peripheral edge portion of the opening. A rubber gasket 33l is placed between the shallow cupshaped member 328 of the microphone subcasing ing the subcasing for the microphone unit. The

lsubcasing for the microphone 596 comprises the front cup-shaped member 328 and the rear cupshaped member 321, the latter cup-shaped member being the portion to which the microphone [04 and the acoustical material is mounted be 'fore the subcas ng is inserted and secured in place in the main casing 355. A detailed description of the various elements mounted with-- in the subcasing comprising the members 32? and 328 will be given subsequently in connection with Figs. 3 to 7, inclusive.

Referring again to Fig. 2. it will be noted that in order to mount the casing 32? in the main fcasing 303, a plurality of L-shaped mounting brackets 329 are secured to the rear cup-shaped casing member 321 in spaced relation around the outside surface thereof. The L-shaped brackets are each provided with an opening into which :a' soft rubber grommet 338 is inserted so that the annular recess of the grommet snugly engages the adjacent surfaces of the said opening. A mounting screw 3%, provided with a metal washer 339a and a locking washer 33%, extends through the central opening in the grommet and is threaded into a tapped hole provided. in

an associated lug-shaped member 3-3? in the casing 30B. Preferably three such cushion type mount ngs are provided to secure the subcasing 321-328 and the microphone unit Hi l in proper relationship with the opening 365. Furthermore,

the cushion type mounting, referred to above, in combination with the soft rubber gasket 33! on gaging the peripheral edge of the opening tilt, is utilized to permit the microphone unit and its associated casing to be tightly clamped in position and thereby prevent any sound waves from reaching the rear surface of the microphone diaphragm.

The individual microphone un t HA is identical with the loudspeaker unit with the eraception that the diaphragm for the microphone IN is provided with an oil silk ring cemented to the periphery of the moulded fiber cone portion whereas the diaphragm for the loudspeaker M35 is a single unitary structure of moulded fiber,

'is illustrated in Figs. 4 and 5, respectively. Re ferring now to Fig. 3, a description will be given of the various structural elements forming the microphone unit HM. It should be understood,

however, that this description also applies to the A 00 formed of Alnico, a bottom plate 'lti, a can ter pole piece 402, and a top plate M3. The parts pole piece till.

drawings is used. verted dome-shaped port on 435 of the diaphragm is formed of moulded fiber, the peripherythereof b sing cemented to an annular ring 4051) of oil silk 6" 40 L582 and 483 are prefer'ablyformed'of Alle' gheny electric metal or other highly permeable magnetic material. The center pole piece 402 is provided with a lower portion 402a of reduceddiameter, which extends within a centrally disposed opening provided in the bottom plate 4!. A sweat connection between the side walls of this opening and the sides of the extended por-- tion L dia may be riveted over on the bottom Surface of the plate illi and thus rigidly secure the center pole piece 402 to the bottom plate 40l At its upper end the center pole piece 402 is provided with a portion 49222 of reduced diamstar, which extends within a centrally disposed opening iiida formed in the top plate 4fl3.- As best shown in Fig. 5 of the drawings, the diameter of the opening iiiSa is slightly larger than the diameter of the upper port on 402!) of the The upper end of the center pole piece concentrically di posed within the openlil io whereby an annular air gap is formed bet een the adjacent side walls of the opening and the center pole piece. The magnetic c rcuit assembly lg completed by means of three clamping screws which extend through openings provided in the bottom plate 40! and the top plate hi3, and are threaded into tapped holes drilled in the bottom plate 606 of the diaphragm support basket assembly. These screws serve rigidly to clamp the annular permanent magnet liiii between the top and bottom plates 403 and till and to secure the diaphragm support as.-

sembly in proper alignment with the center pole are welded to the upper surface of the plate 408.

The moving system also includes two paper Washers did and iii between which the peripheral edge of the diaphragm 495 is clamped. The character of the particular diaphragm utilized depends upon whether the device is to be used as a loudspeaker or as a microphone. If the device to be used as a loudspeaker the diaphragm is of one piece moulded construction including inverted dome shaped portion 485, a concentrio, annular corrugated portion, and an outer flat annular ring portion H350; which extends be tween and is cemented to the paper washers all! and iii in the manner illustrated in Fig. 5. If the device is to be used as a microphone the diaphragm construction illustrated in Fig. 4 of the In this construction the inwhich is disposed between and cemented to the paper washers did and il I. The purpose of the latter arrangement is to permit substantially free vibration of the diaphragm element 4&5, particularly at the low frequencies of the operating frequency range, whereby the response of the device at these frequencies is enhanced. With the onmpiece diaphragm arrangement, as utilined in the loudspeaker unit, the diaphragm is considerably stiiier and, accordingly, the response thereof, particularly at low frequencies, is substantially less than when the diaphragm having the peripheral oil silk ring is used, virtue ofthis increased stiffness, low frequency g mme noise. ioomponentskorsound.are pre ented iron: the juncture of .theisealing rin Alil: andtheend producing any substantial response of :the disvice. c

' :A moving voice coil H3 is-cementedto the-lower inverted dome-shaped portion 4.05 of the diaphragm and is concentrically disposed within, the

air gap defined by the upper portion- 102b of the center ,pole' piece 402 and the opening 403a in the-top plate 403. The ends of the fine wire voice coil: maybe connected to flexible stranded leadin wiresin any desired manner, although preferably the! connections are made by'soldering the ends of thevoice coil wires to terminals secnredsto the L-shaped brackets 4.08 of the diahragm support basket assembly, asis illustrated. in theenlarged views shown in.Figs, 6 and. 7.

More specifically, the L-shaped-bracketABis pro-- vided: with :an openingtherein into whichya, pair .ofeinsuiatingwashers teed are inserted having manner as has "been described hereinbefore in connection with the loudspeaker 05, for electrical-v connection with the conductors extending o an outgoing, channelamplifier.

:In order to preserve the correct lateral and axial spacing of the moving coil 3 .in the .air gap of the magnetic circuit; and to provide .an additional support for the diaphragm 405, .a

string supporting arrangement is provided. This arrangement comprises three connected strings M41) and H are formed of fishr line-or thelike angularly spacedopenings cut through the lower portion of the diaphragm member 485 and are joined together, as indicated at M401, within .the

dished cavity of this member. The freeends-of each of the three strings are respectively placed in a slot 458d in the. extended portion 408cof the 'equiangularly spaced L-shaped brackets 408, and

are anchored in place by upsetting .theslotlfifid :to clinch the string therebetween, and are then cemented. Preferably the three strin s 414d,

.4141) and M40 are formed of fish .line or .thelike and it will be understood that by appropriately tensioning the three strings the required support for the moving coil M3 may be obtained.

; 'In, order to seal the annular cavity, .formed between the spaced-apart annular permanent .magnet 4.89 and the center pole pi ce 402.jr0m 'the acoustical cavity adjacent the lower side of the diaphragm H05, thereby to enhanceithe damping of the dLaphragm and thus produce a, more uniform response thereof over-the entire operating frequency range, a sealing ring 418 is pro- 'vided which snugly encloses the upper end portion 40.21) of :the center pole piece 402, and is i clamped to the underside of the top plate 403 by means of a plurality of "assembly screws 419. This, ring is preferably formed of brass or other non-magnetic material and to further insure the complete sealing of the space between the sealing ring 418 and the portion 40.21) of the center pole a ng; a rubber gasketlllfiais cemented at nortion lllzb.

.As pointed out above, each loudspeaker and, microphone unit, as constructed in the manner Just. described, is preferably surrounded by acousticalinsulatingmaterial, such as a plurality .of felt rings, to. prevent the transmission ofsound Waves to or from the rear side of the diaphragm of thehnit. In the arrangement illustrated in Fig. 3,, which is-an exploded View of the micro.- phone and casing therefor shown in the righthand portion of Fig. 2, the cup-shaped casing member 32! constitutes the subcasing for the. nicrophone assembly.

The microphone unit we is mounted in the cup-shaped casing. 327 by a plurality of screws 3,34, one or which is shown in Fig. 2, and for the pu pose of preventing the casing 321 and any other solid structure mechanically connected thereto .from transmitting vibrations, to the. microphone unit, a mounting assembly of the cushion type is provided More specifically, the annular recess provided in the soft rubber groin! met 33 1a snugly engages the adjacent surfaces of the opening formed in the bottom surfacev of the-casing 32-l,-and a mounting screw 334 having .a metal washer 335 and a locking washer .336 ex.- .tends through a central opening provided in the grommet 334a and through the central opening provided inarubber grommet spacer 33 1b and is .threadedinto a tapped hole provided in the, bottom plate 36i of the .microphone unit. Preferably three suchcushion type mountings are pro.- vided to secure the microphone unit its to the casing l.

In order to prevent acoustical coupling .between the rear surface of the microphoneidiaphragmand the surrounding air, and to prevent undesirable resonance effects within the cavity formed by the casing 321, a suitable amount of acoustical insulating material is provided in the cavity and substantially surrounds the micro- .phone unit. More specifically, in. securing the microphone unit, 04 a compressible felt ring 333a,.is,placed between the bottom of the casing '32] and the bottom plate 43| of the microphone,

and a plurality of compressible felt rings 33.31) are placed around the microphone unit HM to completely fill the space between the wall of th unit andthe inside wall of the casing 321. Incompleting the assembly of the microphone .unit 1.04

'in the casing 321, a soft rubber gasket 332 may be cemented to the paper ring .45 l which secures the peripheral edge of the microphone diaphragm on the supporting ring 401 in order to provide a cushion seal between the edge -.of the diaphragm and the cup-shaped casin cover 328. The. open end of the casing. cover 32.8.15. slightly larger than the open nd of the c n 321 and providesa close-fitting cover. for compressing the soft rubs ber gasket 33;, and the felt rings ttaand .3331 The microphone unit I04 and its individual'subcasing may now be mounted in the previously described manner to cooperate with the opening 305 provided in the main casing 300.

A termin l block 3.4. of nsulating at rialha b n provid d. n order to erminate the income i'ng electrical conductors extending between .an

outgoing amplifier (not shown) and the voice coil of the microphone unit 104. These incoming conductors provide the necessary electrical connection between the outgoing channel amplifier and the v e l'le ds of the mi rophone i hragm- More spe ifically; the t nal l k 4 o nprises.aplur li yofterm nals 204 u d h eto in any suitable manner, which are adapted to accommodate the terminal screws 34%. The incoming conductors may be detachably connected to the terminals by means of the terminal screws 3421), and the stranded lead-in wires extending to the diaphragm voice coil leads, which are soldered to the terminals in the diaphragm support assembly, may be electrically connected by soldering the stranded lead-in'wlres to the terminals 342a. The latter leads are threaded through the central opening provided in the soft rubber grommet 344, which snugly engages the adjacent surfaces of the opening formed in thebottom of the casing 321, and are soldered to the terminals 342a as mentioned above. In order to completely seal the cavity formed by'the casing 321 and to prevent sound waves from passing through the central opening in the grommet 344,'the central opening in the grommet may be sealed with sealing wax or the like. In order to provide a suitable mounting for the terminal block upon the surface of the casing32'l, a pair of threaded spacer lugs 34| are allixed to the rear surface of the casing in any suitable manner, such as, for example, by welding or soldering. The screws 343 provided with the usual locking washers may be inserted through appropriate holes provided in the terminal strip 342 and threaded into the tapped holes in the spacers 3M, thereby to rigidly secure the terminal strip 342 in spaced relation upon the rear of the casing 32?.

As shown, the top surface 304 of the casing 300' is provided with an opening into which the dial mechanism 3G? is inserted so that a portion of the bottom surface of the dial 301 rests upon the shoulder portion 345 which is slightly smaller in diameter than the diameter of the dialing mechanism itself. The dialing mechanism is secured in the above opening by means of a plurality of mounting screws 345a and associated locking Washers 346?) (one of which is shown in Fig. 2). As diagrammatically indicated by the dotted line 34?, a portion of the operating mechanism of the dialing mechanism projects through the opening provided in the top surface of the casing 330, but it is so situated that it does not interfere with the removal of the microphone casing 32'! from the main casing 350.

Since the substation instrument isto be utilized in connection with a telephone system, a 5

portion of the telephone apparatus utilized in signaling a called substation may be mounted upon the base plate 3| I. More particularly, this apparatus comprises a condenser 355 and a ringer 352 mounted upon the base plate 3| in the same manner as is described in detail in the Obergfell Patent No. 2,272,474, granted February 10, 1942. Briefly considered, the mounting arrangement for the condenser 355 comprises a bracket member 354, secured to the side of the condenser housing, which has a U-shaped configuration and is mounted upon the cover plate 3|| by means of screws extending through the base plate and threaded into tapped openings provided in the bracket 354, the heads of the screws being effectively countersunk in raised wells formed in the base plate 3H in the manner illustrated. The ringer 352 comprises a field structure including a polarizing permanent magnet 35| and a pair of windings, one of which is indicated at 353. Also embodied in the ringer are a pair of bells, one of which is shown, carried by a bracket 35! and a striker assembly, not shown, of conventional arrangement. For the purpose of preventing the base plate 3H and any other solid structure 'mechanically connected thereto from exerting any damping influence upon the bells of the ring;- er 352 during the operation of the ringer, and also for preventing vibrations created by the ringer from being transmitted through the solid structure of the instrument to either themicrorphone I04 or the loudspeaker H35, a mounting assembly of the cushion type is used for supporting the ringer upon the base plate ill I. This mounting assemblycomprises a plurality of soft rubber grommets and the usual mounting screws which secure the ringer assembly and theasso ciated grommets to the base member 3| in the manner described in the above-mentioned Obergfell patent. For the purpose of permitting sound waves developed during the operation of the ringer 352 to be transmitted through the outside casing 300, a plurality of rectangularly shaped slots 38-3 are provided in evenly spaced-apartrelation around the lower perpendicular side walls of the casing 380. In order to prevent foreign material and dust from being lodged within the casing 300, the slots 309 may be covered by a fine mesh wire screen or the like which is secured to the interior surface of the casing member 300.

For the purpose of detachably securing the base plate 3H to the casing 363 a plurality. of mounting lugs 358a and 35Gb are moulded or cast as an integral part of the casing and project inwardly from the perpendicular rear and side walls thereof. A third 'lug, not shown, is formed in the opposite side wall of the casing 300'. Each of the mounting lugs is drilled and tapped to receive a threaded portion of the mounting screws, such as screws 349a and 3491). The screws are locked to the base plate 3H and the, heads thereof are effectively countersunk in raised Wells provided in the base plate in the manner illustrated. Screws 349a and 349b are threaded into tapped holes in the mounting lugs 355a and 3501) respectively and serve, together with thenot shown third screw and associated mounting lug,

,tion.

:to clamp the rim of the base plate 3| against the receiving ridge formed in the lower edge of the casing 300. For the purpose of preventing the base plate from contacting the surface ,of the object upon which it is supported, a flexible cushioning element 348 formed of soft rubberis stretchedaround the rim of the base plate 3| I so that a portion 348a thereof extends inwardly around the peripheral portion of the base plate and a portion 348?; thereof is clamped between the base plate and thebottom ridge portion of the casing 389 when thebase plate is mounted upon the casing by the screws349a and 34%. 1 While one embodiment of the invention has been disclosed, it will be understood that various modifications may be made thereinwhichare within the true spirit and, scope of the inven- What is claimed is: l l a main casing having a plurality of openings therein, a loudspeaker unit, a microphone unit, a subcasing for each of said units included within said main casing, a removable subcasing cover supporting said loudspeaker unit within said loudspeaker subcasing with the diaphragm of said loudspeaker adjacent one of said openings, means supporting said microphone unit within said microphone subcasing with its diaphragm adjacent the other of said openings, and a removable base cover supporting said main casing.

. 2. A loudspeaking telephone instrument comprising a housing having a first opening in one 

